In the field of microfabrication represented by the manufacture of integrated circuit devices, lithographic technology enabling microfabrication with a line width of about 100 nm or less using an ArF excimer laser (wavelength: 193 nm), an F2 excimer laser (wavelength: 157 nm), and the like has been demanded in order to increase the degree of integration in recent years. As a radiation-sensitive resin composition applicable to the excimer laser radiation, a number of chemically-amplified radiation-sensitive compositions utilizing a chemical amplification effect between a component having an acid-dissociable functional group and an acid generator, which is a component generating an acid upon irradiation, have been proposed. For example, a high molecular-weight compound for a photoresist comprising a resin component with a specific structure which contains a monomer unit having a norbornane ring derivative as a resin component is known (Patent Document 1 and Patent Document 2).
As a positive-tone photosensitive resin composition suitable for use with an exposure light source with a wavelength of 250 nm or less, particularly 220 nm or less, a resin in which an acid generating group, an alicyclic group, and an acid-dissociable group are introduced into the same molecule (Patent Document 3) and a photosensitive resin composition containing a sulfonium or iodonium salt resin which has a counter anion in the polymer chain in order to increase a photolysis efficiency (Patent Document 4) are known.
However, to achieve a higher degree of integration in the field of semiconductor, a radiation-sensitive resin composition used as a resist is required to possess more excellent resolution. In addition, along with the progress of microfabrication, there is a growing demand for wider focal depth allowance (hereinafter referred to as “DOF”) and narrower line edge roughness (hereinafter referred to as “LER”) of patterns. Along with progress of miniaturization in the semiconductor industry, development of a radiation-sensitive resin composition having excellent resolution and satisfying the demand of wide DOF and narrow LER is urgently need.
The resolution of the projection optical system provided in the projection aligner increases as the exposure wavelength used becomes shorter and the numerical aperture of the projection optical system becomes greater. Therefore, the exposure wavelength, which is a wavelength of radiation used in the projection aligner, has been reduced along with scaling down of integrated circuits year by year, and the numerical aperture of the projection optical system has been increased.
Depth of focus is as important as resolution in exposure. Resolution R and depth of focus δ are respectively shown by the following formulas.R=k1·λ/NA  (i)δ=k2·λNA2  (ii)wherein λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k2 and k2 are process coefficients. A short wavelength is advantageous to provide high resolution R, and a long wavelength is advantageous to provide a larger depth of focus δ.
A photoresist film is formed on the surface of an exposure target wafer, and the pattern is transferred to the photoresist film. In a general projection aligner, the space in which the wafer is placed is filled with air or nitrogen. When the space between the wafer and the lens of the projection aligner is filled with a medium having a refractive index of n, the numerical aperture NA is shown by the following formula.NA=n·sin θ  (iii)wherein θ is an angle shown in FIG. 2. FIG. 2 shows the manner in which light is refracted with a lens, wherein 3 indicates a lens, 4 is an optical axis, 5 is a sample, and 6 shows the direction to which the light moves. D indicates a working distance.
From the formula (iii), it can be understood that a larger NA can be provided by using a liquid with n>1, whereby a high resolution R can be provided for the reason mentioned above.
Such a projection exposure method in which the space between a wafer and the lens of a projection aligner is filled with a medium with a refractive index n to transfer a more minute pattern is called a liquid immersion lithographic method. The liquid immersion lithographic method is considered to be an essential technology for lithography with reduced dimensions, particularly for lithography with dimensions of several ten nanometers.
In the liquid immersion lithography, a photoresist film applied and formed on a wafer and the lens of the projection aligner respectively come into contact with an immersion medium such as water. The immersion medium may permeate the photoresist film and reduces the photoresist resolution. Another problem is elution of a photoresist component into the immersion medium and pollution of the lens surface with such a photoresist component.
For these reasons, a photoresist film is demanded to maintain excellent liquid immersion resistance, without being eluted into an immersion medium such as water during the liquid immersion lithographic method, and the exposed area thereof is required to be easily dissolved in an alkaline solution used as a developer.
However, no radiation-sensitive resin composition has been produced which can produce a stable film against an immersion medium such as water during the liquid immersion lithographic method, exhibits excellent resolution, elutes only a minimal amount of acid generators, and produces an excellent pattern profile in the liquid immersion lithographic method.
[Patent Document 1] JP-A-2002-201232
[Patent Document 2] JP-A-2002-145955
[Patent Document 3] JP-A-10-221852
[Patent Document 4] JP-A-9-325497